> So, my first approach was trying to simplify that, because nonzero_bits > on the subreg operand say that no bits outside of QImode may be non-zero, > both the zero_extend and subreg can be dropped. That is the simplify-rtx.c > change. Then I've figured out that combine.c doesn't actually attempt to > simplify this anyway, so that is the combine.c change. And lastly an i386 > pattern was needed anyway. I've also attempted to simplify: > (zero_extend:SI (subreg:QI (and:DI <something> (const_int 63)) 0)) > into > (subreg:SI (and:DI <something> (const_int 63)) 0) (very small change in > simplify-rtx.c, just drop the requirement that zero_extend mode is as wide > or wider than SUBREG_REG's mode, and when it is <= use gen_lowpart_no_emit > instead of just returning the SUBREG_REG, but that unfortunately regressed > the test1 case, we'd need some further i386.md tweaks. > While in theory this folding looks like a useful simplification, because > of this I'm wondering if other backends don't rely on those actually not > being simplified. > > So, I've as an alternative implemented also an i386.md only fix. > > Thus, do we want the first patch, or first patch + also the > above described further simplify-rtx.c change + some further i386.md tweaks, > or just the second patch instead?
Given the look of the pattern in the second patch, I think we definitely want to simplify upstream. The first simplification looks very natural to me, the further tweaked one less so, so let's go for the first patch alone, with a small tweak: + /* (zero_extend:M (subreg:N <X:O>)) is <X:O> (for M == O) or + (zero_extend:M <X:O>), if X doesn't have any bits outside of N mode + non-zero. E.g. "if X doesn't have any non-zero bits outside of mode N." -- Eric Botcazou
